The invention relates to a process for producing pressure-sensitive adhesive tapes, an acrylate hotmelt pressure-sensitive adhesive being coated from the melt onto a backing and subsequently crosslinked.
In the field of pressure-sensitive adhesive (PSA) compositions, ongoing technological developments in the coating process mean that there is a continual need for new developments. Within the industry, hotmelt processes with solventless coating technology are of increasing importance for the preparation of PSAs, since the environmental strictures are becoming ever greater and the prices of solvents continue to rise. The consequence of this is that solvents are to be eliminated as far as possible from the manufacturing process for PSA tapes. The associated introduction of hotmelt technology is imposing ever greater requirements on the adhesives. Acrylate PSAs in particular are the subject of very intensive investigations aimed at improvements. For high-level industrial applications, polyacrylates are preferred on account of their transparency and weathering stability. As well as these advantages, however, these acrylate PSAs must also meet exacting requirements in respect of shear strength and bond strength. This profile of requirements is achieved by polyacrylates of high molecular weight and high polarity, with subsequent efficient crosslinking. These polar PSAs of high shear strength, however, possess the disadvantage that they are not very suitable for the hotmelt extrusion process, since the high flow viscosity necessitates high application temperatures and, moreover, the molecular weight of the polymer is reduced as a result of shearing within the extruder. Such damage significantly lowers the technical adhesive level. The bond strength and the tack are generally low, since owing to the polar fractions in the adhesives the glass transition temperature is relatively high. There is a marked fall in particular in the shear strengths of hotmelt-coated acrylate PSAs, in comparison to the original solvent-coated PSA. At the present time, therefore, various concepts are being investigated with the aim of reducing the flow viscosity and hence facilitating the extrusion coating of these PSAs.
One very important concept is the targeted adjustment of the molecular weight distribution for the purpose of improved processing properties. Bimodal molecular weight distributions assist easier processing, since low molecular weight fractions lower the flow viscosity while high molecular weight fractions increase the shear strength. Bimodal molecular weight distributions are generally produced by means of targeted blending.
U.S. Pat. Nos. 4,619,979 and 4,843,134 describe a preparation process for the solventless polymerization of acrylates. There, in a particular polymerization reactor, highly branched polymers were prepared. These polymers likewise generally possess a reduced flow viscosity as compared with their linear counterparts.
A further disadvantage of existing acrylate hotmelt PSAs, moreover, is the limited crosslinking mechanism: in comparison to existing solvent systems, these PSAs cannot generally be crosslinked thermally, since otherwise they would gel during processing prior to coating.